Tornadoes and Windstorms/Derechos

A tornado is a violent whirling wind characteristically accompanied by a funnel shaped cloud extending down from a cumulonimbus cloud that progress in a narrow, erratic path. Rotating wind speeds can exceed 300 mph and travel across the ground at average speeds of 25-30 mph. A tornado can be a few yards to about a mile wide where it touches the ground, however, an average tornado, is a few hundred yards wide. It can move over land for distances ranging from short hops to many miles, causing great damage wherever it descends. The funnel is made visible by the dust sucked up and condensation of water droplets in the center of the funnel.

High winds can result from thunderstorms inflow and outflow, or downburst winds when the storm cloud collapses, and can result from strong frontal systems, or gradient winds (high or low pressure systems) moving across the region.  High winds are defined as speeds exceeding 64 knots (73 mph) or greater, either sustaining or gusting.

Downdraft winds are from strong thunderstorm downburst which causes damaging winds on or near ground, and can extend to as little as 2 ½ miles or extend over a hundred miles.  Downdraft wind speeds can be from 80 mph up to 168 mph, and occur quite suddenly as a thunderstorm cloud collapses.  This is different from the winds associated with tornadoes.  Winds associated with storms are convective.  Non-convective winds are caused by fronts or gradient winds.  These speeds can range from light breezes to sustained speeds of 80 to 100 mph.  It is often difficult to separate windstorms and tornado damage when winds get above 64 knots.

Generally the destructive path of a tornado is only a couple hundred feet in width, but stronger tornadoes can leave a path of devastation up to a mile wide. Normally a tornado will stay on the ground for no more than 20 minutes; however, one tornado can touch ground several times in different areas.  Unlike tornadoes, windstorms may have a destructive path that is tens of miles wide and several hundred miles long.

In Iowa most tornadoes occur in the spring and summer months, but twisters can and have occurred in every month of the year. Late afternoon to evening hour tornadoes are the most common, but they can occur at any time of the day.  Iowa has had 1404 tornado events from 1983-2008. The largest single event took place on April 11, 2001 with 28 tornadoes. Our biggest year was 2004 with 120 tornadoes. From 1983-2008, seven Presidential Declarations of Major Disaster have been declared in Iowa that were related to tornadoes out of 1964 total events.

Between 1950 and 2008 Linn County experienced 55 tornadic events which resulted in twenty three injuries and thirty two million dollars in property damage.  During the same time period there were 23 high wind events in Linn County which resulted in almost $2 million dollars in property damage.

Those most at risk from tornadoes and windstorms include people living in mobile homes, campgrounds, and other dwellings without secure foundations or basements.  People in automobiles are also very vulnerable to twisters. The elderly, very young, and the physically and mentally handicapped are most vulnerable because of the lack of mobility to escape the path of destruction. People who may not understand watches and warnings due to language barriers are also at risk.